You stated that your engine races and stop racing continuously. Your engine is very likely surging, due to your carburetor starving for fuel. First, check to see if you are getting a continuous flow of fuel from your fuel tank, to your carburetor. If so, then you likely have either a clogged jet, or clogged fuel passage inside your carburetor.

You will need to give your carburetor a really good cleaning. You can use a small wire, and some carburetor cleaner in a can, to clean the small jets, and fuel passages inside the body of your carburetor. In most cases, it is best to soak your carburetor, prior to cleaning it. If you have an air compressor, follow up with some compressed air, after rinsing your carburetor thoroughly.